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| United States Patent |
6,124,066
|
|
Kim
, et al.
|
September 26, 2000
|
Carrier for electrophotography, an electrostatic latent image developer
and an image forming method
Abstract
A carrier for electrophotography having a resin coating layer containing a
conductive powder having an aspect ratio of not less than 3 on the core
material, in which the dynamic electric resistance of the core material in
a magnetic brush state under an electric field of 10.sup.4 V/cm is not
higher than 1 .OMEGA..cm, and the electric resistance of the resin coating
layer is within a range of from 10 to 1.times.10.sup.8 .OMEGA..cm, and a
developer for an electrostatic latent image using the carrier. An image
forming method on which the developing process includes using the
above-mentioned developer for an electrostatic latent image, whose
development curve, expressed by a contrast potential and the developing
toner quantity, has a saturation area, and applying the developing bias to
the developer carrying member so that the developing toner quantity shows
the saturation characteristic.
| Inventors:
|
Kim; Suk (Minami-Ashigara, JP);
Yamamoto; Yasuo (Minami-Ashigara, JP);
Yanagida; Kazuhiko (Minami-Ashigara, JP)
|
| Assignee:
|
Fuji Xerox Co., Ltd. (Tokyo, JP)
|
| Appl. No.:
|
993666 |
| Filed:
|
December 18, 1997 |
Foreign Application Priority Data
| Dec 24, 1996[JP] | 8-344143 |
| Feb 25, 1997[JP] | 9-040991 |
| May 06, 1997[JP] | 9-115914 |
| Current U.S. Class: |
430/111.32; 430/111.35; 430/122 |
| Intern'l Class: |
G03G 009/113 |
| Field of Search: |
430/106.6,108,111,122
|
References Cited
U.S. Patent Documents
| 5391451 | Feb., 1995 | Yoshie et al. | 430/106.
|
| 5576133 | Nov., 1996 | Baba et al. | 430/106.
|
| 5654120 | Aug., 1997 | Hakata et al. | 430/106.
|
| 5683844 | Nov., 1997 | Mammino | 430/106.
|
| 5686182 | Nov., 1997 | Maniar | 428/404.
|
| 5853877 | Dec., 1998 | Shibuta et al. | 478/357.
|
| 5932387 | Aug., 1999 | Yamamoto et al. | 430/108.
|
| Foreign Patent Documents |
| 61-107257 | May., 1986 | JP | .
|
| 61-130959 | Jun., 1986 | JP | .
|
| 6-161157 | Jun., 1994 | JP | .
|
| 7-120086 | Dec., 1995 | JP | .
|
Primary Examiner: Chapman; Mark
Attorney, Agent or Firm: Oliff & Berridge, PLC
Claims
What is claimed is:
1. A carrier for an electrostatic latent image developer for
electrophotography, having a resin coating layer containing a conductive
powder on a core material, wherein the aspect ratio of said conductive
powder is not less than 3, the dynamic electric resistance of said core
material in a magnetic brush state under an electric field of 10.sup.4
V/cm is lower than 1 .OMEGA..cm, and the electric resistance of said resin
coating layer is within a range of from 10 to 1.times.10.sup.8 .OMEGA..cm.
2. A carrier for an electrostatic latent image developer according to claim
1, wherein said conductive powder is contained in an amount of from 2 to
20% by volume with respect to the resin coating layer.
3. A carrier for an electrostatic latent image developer according to claim
1, wherein the film thickness of the resin coating layer is from 0.3 to 5
.mu.m.
4. A carrier for an electrostatic latent image developer according to claim
1, wherein the average carrier particle diameter is in a range of from 10
to 100 .mu.m.
5. A carrier for an electrostatic latent image developer according to claim
1, wherein the core material is ferrite.
6. A carrier for an electrostatic latent image developer according to claim
5, wherein the electric resistance of the carrier is in a range of from 10
to 1.times.10.sup.9 .OMEGA..cm.
7. A carrier for an electrostatic latent image developer according to claim
1, wherein the electric resistance of said conductive powder is not higher
than 1.times.10.sup.6 .OMEGA..cm.
8. A carrier for an electrostatic latent image developer according to claim
7, wherein the conductive powder is one of a conductive metal oxide or a
powder coated with a conductive metal oxide.
9. A carrier for an electrostatic latent image developer according to claim
1, wherein the conductive powder is one of a conductive metal oxide or a
powder coated with a conductive metal oxide.
10. An electrostatic latent image developer comprising:
toner particles comprising a binder resin and a coloring agent, and
a carrier having a core material and a resin coating layer provided on the
core material,
wherein the resin coating layer of the carrier (i) contains a conductive
powder having an aspect ratio of not less than 3, and (ii) has an
electrical resistance in a range of from 10 to 1.times.10.sup.8
.OMEGA..cm,
the core material of the carrier has a dynamic electric resistance of less
than 1 .OMEGA..cm under an electric field of 10.sup.4 V/cm in a magnetic
brush state.
11. An electrostatic latent image developer according to claim 10, wherein
the electric resistance of said conductive powder is not higher than
1.times.10.sup.6 .OMEGA..cm.
12. An electrostatic latent image developer according to claim 11, wherein
the conductive powder is one of a conductive metal oxide or a powder
coated with a conductive metal oxide.
13. An electrostatic latent image developer according to claim 10, wherein
the conductive powder is one of a conductive metal oxide or a powder
coated with a conductive metal oxide.
14. An image forming method comprising the steps of:
forming a latent image on a latent image carrying member, developing said
latent image by using a developer, transferring the developed toner image
to an image receiving member, and thermally fixing the toner image on the
image receiving member,
wherein said method uses an electrostatic latent image developer
comprising:
toner particles comprising a binder resin and a coloring agent, and
a carrier having a resin coating layer provided on the core material, said
resin coating layer (i) containing a conductive powder which has an aspect
ratio of not less than 3, and (ii) having an electric resistance in a
range of from 10 to 1.times.10.sup.8 .OMEGA..cm, said core material having
a dynamic electric resistance of less than 1 .OMEGA..cm under an electric
field of 10.sup.4 V/cm in a magnetic brush state.
15. An image forming method according to claim 14, wherein the developing
process uses an electrostatic latent image developer whose development
curve, expressed by a contrast potential defined by the development bias
potential and the potential of the exposed portion of the latent image
carrying member, as well as the quantity of the developing toner
transferring to the latent image on the latent image carrying member has a
saturation area, and the developing bias is applied to the developer
carrying member so that the quantity of the developing toner shows the
saturation characteristic.
16. An image forming method according to claim 14, wherein the developing
bias potential is a developing bias potential in which the
alternating-current electric field, having a voltage between peaks of from
100 to 500 V and a frequency of from 400 Hz to 20 kHz, is superposed on
the direct-current electric field.
17. An image forming method according to claim 14, wherein the electric
resistance of said conductive powder is not higher than 1.times.10.sup.6
.OMEGA..cm.
18. An image forming method according to claim 17, wherein the conductive
powder is one of a conductive metal oxide or a powder coated with a
conductive metal oxide.
19. An image forming method according to claim 14, wherein the conductive
powder is one of a conductive metal oxide or a powder coated with a
conductive metal oxide.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a carrier for the electrostatic latent
image developers, an electrostatic latent image developer and an image
forming method used for developing an electrostatic latent image formed by
electrophotography and electrostatic recording methods and the like.
Furthermore, the present invention relates to an image forming method
executed by a image forming apparatus such as a digital printer, a digital
copying machine and the like which handles an image as a digital signal.
2. Description of the Related Art
The methods to visualize the image information via the electrostatic latent
image, such as electrophotography, have been utilized in various fields.
In electrophotography, an electrostatic latent image is formed on a
photosensitive material in the charging and the light exposure processes,
and the electrostatic latent image is developed by a developer containing
a toner, and visualized through the transfer and the fixing processes.
On the other hand, in the digital image forming apparatus, binary
information of ON/OFF is given as two-dimensional information of
predetermined place on the photosensitive material based on the character
and image data. When a halftone image is recorded by using such method, an
area modulation method using a halftone dot structure and multiple line
structure has been conventionally adopted in various printers and copying
machines of the digital electrophotographic method, since the algorithm is
relatively simple and of low cost.
Furthermore, in the image forming apparatus which reproduces a multi-tone
image by using the electrophotographic method, in particular, in a color
image forming apparatus, there are two-component developers comprising a
toner and a carrier, and one-component developers using a magnetic toner
or the like singly, as the developer. The two-component developer has been
widely used since the carrier fulfills the functions of stirring,
transferring and charging of the developer, thus functions are separated
in the developer, and stable charging characteristics and good
controllability can be obtained.
Furthermore, as the developing method, a cascade method had been used
before, but recently a magnetic brush method which uses a magnetic roll as
a developer transfer carrying member has become the mainstream. As the
two-component magnetic brush development method, a conductive magnetic
brush (CMB) development method using a conductive carrier and an
insulating magnetic brush (IMB) development method using an insulating
carrier are known.
The IMB development is characterized by the relationship between the latent
image potential and the image density on the photosensitive material is
linear and the gradient is small, but on the other hand, the solid portion
is not sufficiently filled and the edge effect is large. On the other
hand, the CMB development has characteristics that it does not have the
edge effect and the solid portion is sufficiently filled, in contrast to
the insulating magnetic brush developing method, but the relationship
between the latent image potential and the image density is abrupt and the
gradient is large, and has such defects that carrier-over (migration of
the carrier toward the photosensitive material) and brush marks due to the
break of the latent image caused by a bias leak are easily caused.
These problems do not have much influence on visual image quality, when
black and white images are formed by using only a black toner, and if the
degree of the defects is soft. When color images are formed by superposing
color toners, however, these problems will become fatal defects. In the
black and white images, the above problems are caught only as a
microscopic change in density, while in the color images, the above
problems are caught as a microscopic change in hue, and noises having
different colors will exist in the gradation image. Accordingly, the above
problems have an extremely bad effect on the quality of the visual image,
particularly of a color image.
There have been disclosed some methods to improve these points and obtain a
conductive magnetic brush in which the edge effect is small, and
carrier-over and brush marks are hardly caused.
For example, in Japanese Patent Application Publication (JP-B) No.
7-120,086, there is disclosed a carrier in which a core material having a
relatively low electric resistance (hereinafter referred to as a "carrier
core", or simply a "core") is coated with a resin having high resistance,
thereby the electric resistance abruptly changes in some electric field,
and it has high resistance in a low electric field, and low resistance in
a high electric field. According to the disclosure thereof, since the
latent image portion has a high electric field and the non-latent image
portion has a low electric field, an excellent solid black printing can be
obtained while carrier-over in the non-latent image portion is not caused.
In this invention (according to the description in the examples and
effects in the publication Japanese Patent Application Publication (JP-B)
No. 7-120,086), however, it is presumed that the film thickness of the
resin coating layer is considerably thin, and cores having low resistance
are exposed partially. Hence, it is considered that the resistance is low
in the high electric field due to this structure. Actually, as in the
Comparative Examples described later, the electric resistance of a carrier
whose core material is coated completely with a resin coating layer having
a thick film thickness is high even in a high electric field, and a good
solid image could not be obtained. With the partially coated carrier
described above in which a part of the core material having low resistance
is exposed, the electric charge moves easily via the exposed face, hence
brush marks are easily caused in the latent image portion.
Furthermore, in Japanese Patent Application Laid-Open (JP-A) Nos.
61-107,257 and 61-130,959, there is disclosed a ferrite having relatively
low electric resistance and having unevenness based on the primary
particles on the surface. According to the disclosure thereof, the leak
between electric charges having different polarity is suppressed, and
brush marks are prevented because of the ferrite having such a minute
unevenness. However, since it has a minute unevenness on the carrier
surface, the contact area with the toner increases, resulting in a problem
that the toner easily adheres, and its charge-imparting ability as a
carrier is deteriorated with the lapse of time.
Furthermore, in Japanese Patent Application Laid-open (JP-A) No. 6-161,157,
there is described a carrier defined by a ratio of the current value of
the carrier core and the current value of the whole carrier coated with a
resin, and hence the resolution, density of the solid image and the fine
line reproducibility can all be satisfied at the same time. However,
sufficient effect is not found in the prevention of the image defects in
color images, in particular.
In addition, there is found no report in either example described above
that the stability against the environmental changes is improved.
As described above, with regard to the CMB development, the improvement is
not sufficient from the standpoint of the recent demand for the high image
quality, including color images.
SUMMARY OF THE INVENTION
The object of the present invention, therefore, is to provide a carrier
which does not cause image defects such as brush marks, carrier-over or
the like, but can obtain good solid images, and has high stability against
the environmental changes as well as durability, in images obtained by the
electrostatic latent image development method, in particular color images
and a developer and an image forming method using the carrier.
Furthermore, in view of the above conventional drawbacks, it is an object
of the present invention to provide an image forming method and an image
forming apparatus in which even if the sensitivity of the photosensitive
material is uneven and there are environmental changes, the quantity of
the developing toner migrating toward the latent image is stable, the
solid portions are sufficiently filled, and the edge effect, brush marks
and carrier-over can be prevented.
The present inventors have investigated with a view to attain the
above-mentioned objects, and have found that in order to obtain good solid
images by preventing image defects such as brush marks, carrier-over and
the like, it is necessary that the electric resistance of the carrier is
within a predetermined range, therefore it is important that the electric
resistance of the carrier core is lower than the predetermined value and
the electric resistance of the resin coating layer is within a
predetermined range, and that the stability against the environmental
changes is improved by using an acicular conductive powder as the
conductive powder contained in the resin coating layer. Thus, the present
inventors completed the present invention.
Furthermore, the present inventors have investigated with a view to solving
the above-mentioned problems, and found that the shape and the content of
the conductive powder affected the stability against environmental changes
and deterioration with time, and when a acicular conductive powder is
used, excellent stability against environmental changes can be obtained.
Furthermore, it has been found that it is effective for stabilizing the
developing toner quantity to use a developing agent in which the
development curve, expressed by a contrast potential defined by the
development bias potential and the potential of the exposed portion of the
latent image carrying member, and by the quantity of the developing toner
migrating toward the latent image on the latent image carrier, has a
saturation area, and that even if the electric resistance of the
resin-coated carrier per se is the same, there is a difference in the
saturation characteristics depending upon the electric resistance of the
core material, leading to the completion of the present invention.
That is, it is the object of the present invention to provide a carrier for
an electrostatic latent image developer for electrophotography having a
resin coating layer containing a conductive powder on the core material,
wherein the aspect ratio of the conductive powder is not less than 3, the
dynamic electric resistance of the core material in a magnetic brush state
under an electric field of 10.sup.4 V/cm is lower than 1 .OMEGA..cm, and
the electric resistance of the resin coating layer is within a range of
from 10 to 1.times.10.sup.8 .OMEGA..cm.
It is another object of the present invention to provide an electrostatic
latent image developer comprising toner particles comprising a binder
resin and a coloring agent, and a carrier having a resin coating layer
provided on the core material, wherein the carrier has a resin coating
layer containing a conductive powder having an aspect ratio of not less
than 3, and having an electric resistance in a range of from 10 to
1.times.10.sup.8 .OMEGA..cm, the resin coating layer coated on the core
material in a magnetic brush state having a dynamic electric resistance
under an electric field of 10.sup.4 V/cm of less than 1 .OMEGA..cm.
An image forming method comprising the steps of: forming a latent image on
a latent image carrying member, developing said latent image by using a
developer, transferring the developed toner image to an image receiving
member, and thermally fixing the toner image on the image receiving
member,
wherein said method uses an electrostatic latent image developer
comprising:
toner particles comprising a binder resin and a coloring agent, and
a carrier having a resin coating layer provided on the core material, said
resin coating layer containing a conductive powder which has an aspect
ratio of not less than 3 and has an electric resistance in a range of from
10 to 1.times.10.sup.8 .OMEGA..cm, said core material having a dynamic
electric resistance of less than 1 .OMEGA..cm under an electric field of
10.sup.4 V/cm in a magnetic brush state.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a development curve expressed by a contrast potential and the
quantity of the developing toner, and the development curve has a
saturation area.
FIG. 2 is a diagram showing the overall structure of the image forming
apparatus.
FIG. 3 is a structural diagram of a light-beam scanning device used in the
image forming apparatus of FIG. 2.
FIG. 4 is a structural diagram of a pulse-width modulation device used in
the light-beam scanning device of FIG. 3.
FIG. 5 is a schematic structural diagram of the developing section
constituting a rotary developing device used in the image forming
apparatus of FIG. 1.
FIGS. 6A, 6B and 6C show exposure energy profile of the photosensitive
material.
FIG. 7 is a diagram explaining the distance between picture elements.
FIGS. 8A and 8B show the light potential attenuation characteristics of the
photosensitive material.
FIGS. 9A and 9B show potential profiles of the photosensitive material put
into a binary value.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will now be described with reference to preferred
embodiments.
The carrier for an electrostatic latent image developer of the present
invention (hereinafter simply referred to as a "carrier") has a resin
coating layer having a mean resistance of from 10 to 1.times.10.sup.8
.OMEGA..cm formed on a core having low resistance of not higher than 1
.OMEGA..cm. With such a structure, the quality of solid images and
prevention of image defects such as brush marks, carrier-over or the like
can be obtained concomitantly. The reason thereof is presumed as follows.
In general, when a conductive body is placed in an electric field, the
charge is rearranged along the electric field, and a so-called
polarization is caused. The speed of the polarization has a relation with
the electric resistance of the conductive body, and as the electric
resistance becomes low, the speed of the polarization becomes fast. It is
considered that such a phenomenon also occurs inside of the core of the
carrier placed between the developing roll and the photosensitive
material, and that if the electric resistance of the core is low enough to
complete the poralization of the core for about 10.sup.-3 seconds when the
development is carried out, a development electrode effect due to the
polarization of the core itself is added to the charge injection from the
developing roll, and, as a result, good solid images can be obtained.
However, even if the electric resistance of the core is low, if the
electric resistance of the resin coating layer is high, the overall
electric resistance becomes high, and good solid images cannot be
obtained. On the other hand, since the charge injected from the developing
roll flows mainly on the carrier surface, if the electric resistance of
the resin coating layer is too low, brush marks and carrier-over are
easily caused. Therefore, it is considered that the range of the electric
resistance of the core and the resin coating layer fulfilling these
conditions are in the above-mentioned range.
As a carrier core used in the present invention, any of the conventionally
known carrier cores can be used, but particularly preferably, ferrite
having low resistance is selected. As other carrier cores, iron powder,
magnetite and the like are known. In the case of iron powder, a toner and
an externally additive easily adhere to it due to its large specific
gravity, hence iron powder has poorer stability than ferrite. In the case
of magnetite, it has problems in that it is difficult to control the
resistance and the latitude of the electric resistance is narrow. On the
other hand, ferrite can gain low resistance by reduction in a hydrogen
current at a certain temperature after baking, and ferrite having various
electric resistances can be obtained by controlling the amount of hydrogen
ventilation, temperature, the reduction time and the like, hence ferrite
is particularly preferable.
The carrier core used in the present invention has a dynamic electric
resistance which as measured in a form of a magnetic brush in an electric
field of 10.sup.4 V/cm, is lower than 1 .OMEGA..cm. If the electric
resistance of the core material exceeds 1 .OMEGA..cm, the electric
resistance of the whole carrier has to be made low in order to obtain a
saturation area of the amount developing toner, and brush marks due to the
bias leak and carrier-over will be easily caused, hence it is not
preferable.
Here, the saturation area means an area where, as shown in FIG. 1, in a
development curve expressed by a contrast potential defined by the
development bias potential and the potential of the exposed portion of the
latent image carrying member, and the quantity of the developing toner
migrating toward the latent image on the latent image carrying member, the
developing toner quantity migrating to the latent image reaches the limit
when the contrast potential becomes higher than a predetermined value and
the developing toner quantity is hardly changed by the change of the
contrast potential.
In addition, the electric field of 10.sup.4 V/cm is close to the developing
electric field in an actual apparatus, and the above-mentioned electric
resistance is defined by a value under this electric field. The dynamic
electric resistance of the carrier core is determined by the following
manner. A plate electrode having an area of 3 cm.sup.2 is placed opposite
a developing roll having a diameter of 4 cm and a length in the axial
direction of 10 cm with an interval of 2.5 mm, and a carrier core of about
30 cm.sup.3 is placed on the developing roll opposite to the plate
electrode to form a magnetic brush. Voltage is applied between the
developing roll and the plate electrode while rotating the developing roll
at a rotation speed of 120 rpm, and the electric current flowing at that
time is measured. The electric resistance is determined from the obtained
current-voltage characteristics by using an expression of Ohm's law.
Incidentally, it is well known that, in general, there is a relationship
of log J.varies.E1/2 between the applied electric field E and the current
density J at this time (for example, Japanese Journal of Applied Physics,
Vol. 19, No. 12, p 2412.about.). When the electric resistance is
considerably low as in the carrier core used in the present invention,
sometimes measurement becomes impossible because high current flows in the
high electric field of 10.sup.3 V/cm or higher. In such a case,
measurement is performed at three points or more in the low electric
field, and the electric resistance is determined by the extrapolation up
to the electric field of 10.sup.4 V/cm by means of the least square method
by using the above-mentioned relational expression.
The average particle diameter of the carrier core is preferably from 10 to
100 .mu.m, more preferably, from 20 to 80 .mu.m. If the average particle
diameter is smaller than 10 .mu.m, scattering of the developer from the
developing apparatus is easily caused, and if the average particle
diameter is larger than 100 .mu.m, it is difficult to obtain sufficient
image density.
As the resin coating formed on the core, there can be mentioned polyolefin
resins, such as polyethylene, polypropylene; polyvinyl and polyvinylidene
resins, such as polystyrene, acrylic resin, polyacrylonitrile, polyvinyl
acetate, polyvinyl alcohol, polyvinyl butyral, polyvinyl chloride,
polyvinyl carbazole, polyvinyl ether, and polyvinyl ketone; vinyl
chloride/vinyl acetate copolymer; styrene/acrylic acid copolymer, straight
silicone resin comprising organosiloxane bond or modified products
thereof; fluorine resins, such as polytetrafluoroethylene, polyvinyl
fluoride, polyvinylidene fluoride, polychlorotrifluoroethylene; polyester;
polyurethane; polycarbonate; amino resins, such as urea-formaldehyde
resin; and epoxy resins. These resins may be used singly, or in a
combination of two or more of them.
The thickness of the resin coating layer is preferably from 0.3 to 5 .mu.m,
more preferably, from 0.5 to 3 .mu.m. If the thickness of the resin
coating layer is less than 0.3 .mu.m, it is difficult to form a uniform
resin coating layer on the core surface, in particular, when a core having
low resistance is used as in the present invention, migration of charges
via the exposed face is caused, and image defects are easily caused. In
addition, if the thickness of the resin coating layer is larger than 5
.mu.m, granulation between carriers are caused, and a uniform carrier is
not obtained.
As the conductive powder added to the resin coating layer in the present
invention, the one having an acicular form is used. "Acicular" powder
means a powder having a ratio of the long axis (fiber length) to the short
axis (fiber diameter) (long axis/short axis; hereinafter referred to as
"aspect ratio") of not less than 3, preferably not less than 5, more
preferably not less than 10. The acicular conductive powder easily forms a
continuous conductive passage in the resin coating layer, therefore the
added amount thereof can be less than of the conductive powder having a
spherical shape. Many conductive powders have a hydroxyl group present on
the surface and sometimes are porous, hence water is easily adsorbed, and
when the added amount thereof is large, the electric resistance and the
charging properties of the carrier oscillate a great deal due to the
fluctuations in humidity, resulting in various problems. Therefore, by
reducing the added amount of the conductive powder, it is possible to
improve the stability against the fluctuations in humidity. When the
conductive powder is dispersed and mixed in the resin coating, there is a
possibility that the conductive powder is cut in the direction of the
fiber length, and the aspect ratio is decreased. Therefore, as the
conductive powder, preferably the one having the aspect ratio as high as
possible is used, and the one having the aspect ratio within the
above-mentioned range and imparting a desired electric resistance to the
resin coating layer even when the conductive powder is cut may be used
without problem.
The acicular conductive powder has preferably a long axis of from 0.1 to 20
.mu.m. Even if the aspect ratio is not less than 3, if the long axis is
shorter than 0.1 .mu.m, the conductive powder is broken during the process
to disperse into the resin coating to decrease the effect. On the other
hand, if the long axis is longer than 20 .mu.m, the rate at which the
conductive powder protrudes from the resin coating layer surface
increases, hence migration of charges is caused and image defects are
easily caused. The short axis of the conductive powder is preferably from
0.01 to 1 .mu.m. If the short axis is without this range, the
dispersibility is deteriorated, and the properties of the carrier become
uneven. As the added amount of the conductive powder, it is preferably
from 2 to 40% by volume with respect to the resin coating layer, more
preferably from 4 to 35% by volume. If the content of the conductive
powder is less than 2% by volume, the electric resistance of the resin
coating layer does not decrease to the desired value, and if the content
of the conductive powder is larger than 40% by volume, the environmental
stability is deteriorated.
The electric resistance of the acicular conductive powder is preferably not
higher than 1.times.10.sup.6 .OMEGA..cm, respectively. When the electric
resistance exceeds 1.times.10.sup.6 .OMEGA..cm, the desired resistance is
difficult to obtain in the carrier as a whole.
As the material of the acicular conductive powder, it is not particularly
limited so far as it has the desired shape and desired electric
resistance, but for example, a complex substance having surfaces of fine
particles such as titanium oxide, zinc oxide, aluminum borate, potassium
titanate, tin oxide and the like coated with a conductive metal oxide, or
a simple substance of a conductive metal oxide are preferable. Here, as
the conductive metal oxide, there can be mentioned a metal oxide doped
with antimony (for example, antimony doped type tin oxide) and metal
oxides of an oxygen deficit type (for example, oxygen deficit type tin
oxide).
The electric resistance of the resin coating layer is from 10 to
1.times.10.sup.8 .OMEGA..cm, preferably from 10.sup.3 to 10.sup.7
.OMEGA..cm. The electric resistance of the resin coating layer is
controlled by kinds, quantity and the like of the conductive powder and
the resin coating layer to be used. If the electric resistance of the
resin coating layer is smaller than 10 .OMEGA..cm, the charge easily
migrates on the carrier surface, resulting in image defects. If the
electric resistance of the resin coating layer is larger than 10.sup.8
.OMEGA..cm, even if a core having low resistance is used, good solid
images cannot be obtained. A resin coating film having a thickness of from
a fraction of .mu.m to several .mu.m is formed on an ITO conductive glass
substrate using an applicator, and a gold electrode is formed thereon by
vapor deposition to determine the electric resistance of the resin coating
layer from the current-voltage characteristics in the electric field of
10.sup.2 V/cm.
As a method to form a resin coating layer on the carrier core, there can be
mentioned an dipping method in which a carrier core is dipped in a
solution for forming a resin coating layer, a spray method in which a
solution for forming a resin coating layer is sprayed on the carrier core
surface, a fluid bed method in which a solution for forming a resin
coating layer is sprayed to the carrier core suspended by an air flow, and
a kneader coater method in which a carrier core and a solution for forming
a resin coating layer are mixed in a kneader coater and the solvent is
removed.
The solvent used for the solution for forming a resin coating layer is not
particularly limited so far as it dissolves the above-mentioned resin
coating, and for example, aromatic hydrocarbons such as toluene, xylene
and the like, ketones such as acetone, methyl ethyl ketone and the like,
and ethers such as tetrahydrofuran, dioxane and the like can be used.
Furthermore, a sand mill, dino mill, homomixer or the like can be used for
the dispersion of the conductive powder.
As a preferable range of the dynamic electric resistance when a carrier
provided with a resin coat on its surface is measured in a form of a
magnetic brush, it is from 10 to 1.times.10.sup.9 .OMEGA..cm, more
preferably from 1.times.10.sup.3 .OMEGA..cm to 1.times.10.sup.9 .OMEGA..cm
in the electric field of 10.sup.4 V/cm. If the above-mentioned electric
resistance is smaller than 10 .OMEGA..cm, image defects are easily caused,
and if it is larger than 10.sup.9 .OMEGA..cm, good solid images are
difficult to obtain. In addition, the measurement method of the dynamic
electric resistance is similar to that in the carrier core.
The carrier of the present invention is mixed with a toner and used as a
two-component developer. Said toner is obtained, according to customary
procedures, by melting and kneading a coloring agent and other additives
in a binder resin, cooling and pulverizing them and classifying them as
required.
As the binder resin of the toner, there can be mentioned homopolymers or
copolymers of styrenes such as styrene, chlorostyrene and the like;
monoolefins such as ethylene, propylene, butylene, isoprene and the like;
vinyl esters such as vinyl acetate, vinyl propionate, vinyl benzoate,
vinyl acetate and the like; .alpha.-methylene aliphatic monocarboxylic
acid esters such as methyl acrylate, ethyl acrylate, butyl acrylate,
dodecyl acrylate, octyl acrylate, phenyl acrylate, methyl methacrylate,
ethyl methacrylate, butyl methacrylate, dodecyl methacrylate and the like;
vinyl ethers such as vinylmethyl ether, vinylethyl ether, vinylbutyl ether
and the like; vinyl ketones such as vinylmethyl ketone, vinylhexyl ketone,
vinyl isopropenyl ketone and the like. As representative binder resins,
there can be mentioned polystyrene, styrene-acrylate copolymer,
styrene-methacrylate copolymer, styrene-acrylonitrile copolymer,
styrene-butadiene copolymer, styrene-maleic anhydride copolymer,
polyethylene and polypropylene. In addition, there can be mentioned
polyester, polyurethane, epoxy resin, silicon resin, polyamide, modified
rosin, paraffin, and waxes.
As the coloring agents, there can be exemplified as representatives, carbon
black, nigrosine, aniline blue, chalcoil blue, chrome yellow, ultramarine
blue, DuPont oil red, Quinoline yellow, methylene blue chloride,
Phthalocyanine Blue, Malachite green-oxalate, lamp black, Rose Bengal, C.
I. Pigment.Red48:1, C. I. Pigment.Red122, C. I. Pigment.Red57:1, C. I.
Pigment.Yellow 97, C. I. Pigment.Yellow 12, C. I. Pigment.Blue 15:1, C. I.
Pigment.Blue15:3.
The toner may contain known additives such as charge-controlling agents,
fixing auxiliaries or the like as required. The average particle diameter
of the toner is not larger than 30 .mu.m, preferably from 4 to 20 .mu.m.
As the ratio of the toner when a toner and a carrier are mixed to prepare a
developer, it is preferably from 0.3 to 30% by weight with respect to the
whole developer. Furthermore, silica, alumina, tin oxide, strontium oxide,
various resin powders, and other known externally added agents can be
mixed in order to improve the flowability of the developer.
The developer thus obtained can be used in an image forming method
comprising a process to form a latent image on a latent image carrier, a
process to develop the latent image by using a developer, a process to
transfer the developed toner image to a image receiving member, and a
fixing process to heat and fix the toner image on the image receiving
member.
According to the present invention, preferably, in an image forming method
having a latent image forming process for forming a latent image on a
latent image carrying member charged uniformly based on the image data and
a developing process for developing the latent image with a developer
supported on a developer carrying member to which a developing bias
potential is applied, or in an image forming apparatus having a latent
image forming means for forming a latent image on a latent image carrying
member charged uniformly based on the image data and developing means for
developing the latent image with a developer supported on a developer
carrying member to which a developing bias potential is applied by bias
applying means, the above-mentioned developer of the present invention is
used, and a developing bias voltage is applied to the developer carrying
member so that the quantity of the developing toner migrating to the
latent image shows saturation characteristics.
Here, saturation characteristic means that the developing toner quantity is
hardly changed by the change of the contrast potential defined by the
development bias potential applied to the developer carrying member and
the potential of the exposed portion of the latent image carrying member.
As shown in FIG. 1, when the contrast potential in which the gradient of
the development curve becomes 1/5 or less compared to the gradient at the
initial stage of the development is designated as Vs, the development bias
potential can be applied to the developer carrying member, so that the
value, obtained by subtracting an absolute value .vertline.V1.vertline. of
the average surface potential V1 of the photosensitive materials of the
exposed portion when the exposure of the input image area factor is 100%,
from the absolute value .vertline.V bias.vertline. of the development bias
potential V bias, becomes larger than .vertline.Vs.vertline.. Thus, by
setting the development bias potential, even if there is a sensitivity
difference in the photosensitive material, the developing toner quantity
becomes stable and reproduction of good images becomes possible.
Specifically, it is preferred to use the development bias potential in
which the alternating-current electric field having a voltage between
peaks of from 100 to 500 V, and a frequency of from 400 Hz to 20 kHz, is
superposed on the direct-current electric field.
FIG. 2 shows one embodiment of the image forming apparatus to which the
present invention is applied.
This image forming apparatus 10 includes a control section 12 for
controlling the whole image forming apparatus 10; an original reading
section 14 which radiates a light to the original to prepare image signals
for each color from the light transmitted through the original or
reflected from the original; a photosensitive material 16 as a latent
image carrying member rotating in the direction of an arrow A; a charging
device 18 arranged in the vicinity of the photosensitive material 16 for
uniformly charging the photosensitive material 16; a potential sensor 20
arranged downstream in the rotation direction of the charging device 18
for measuring the potential of the charged photosensitive material 16; a
light-beam scanning device (ROS) 22 for scanning and exposing the
photosensitive material 16 charged in the exposure section 21 formed
upstream of the potential sensor 20 in the rotation direction, based on
the image data from the original reading section 14, to form a latent
image; a rotary developing device 24 arranged downstream of the exposure
section 21 in the rotation direction for transferring a toner to the
latent image to form a visible image; a transfer device 26 arranged
downstream of the rotary developing device 24 in the rotation direction
for transferring the visible image to a recording material; a cleaner 28
arranged downstream of the transfer device 26 in the rotation direction
for removing the toner remaining on the photosensitive material 16; a
pre-exposure device 30 for exposing the photosensitive material 16 to
remove the residual potential; and a fixing device 32 for fixing the
visible image on the recording material.
The original reading section 14 includes a light source (not shown) for
radiating a light to the original, a color filter (not shown) for
separating the light transmitted through or reflected from the original
into each color, a photoelectric converter (not shown) for converting the
strength of the light for each color into an electric signal which is
analog data, an A/D (analog-to-digital) converter (not shown) for
converting the electric signal for each color into an image signal for
each color which is digital data, and a memory (not shown) for storing the
image signal for each color, and the image signal stored in the memory is
output to the light-beam scanning device 22 sequentially for each color
based on the signal from the control section 12.
As shown in FIG. 3, the light-beam scanning device 22 includes a
semiconductor laser 34 for radiating the laser beam 38, a pulse-duration
modulation device 36 for switching ON/OFF of the semiconductor laser 34
based on the image signal from the original reading section 14, a
collimator lens 40 for collimating the laser beam 38 radiated from the
semiconductor laser 34 to the parallel beam, apolygon mirror 42 for
deflecting the parallel beam from the collimator lens 40 toward the
photosensitive material 16 at an equiangular rate, an f.theta. lens 44
arranged between the polygon mirror 42 and the photosensitive material 16
for forming a beam spot of a predetermined size on the photosensitive
material 16, and a sensor 46 for forming a scanning start signal for
generating an SOS signal for detecting the light scanning start timing.
As shown in FIG. 4, the pulse-width modulation device 36 includes a D/A
converter 48 for converting the image signal which is digital data from
the original reading section 14 into an electric signal which is analog
data, a sawtooth generator 50 for forming a number of sawtooth waveforms
of different frequency, a waveform selecting circuit 52 for selecting the
sawtooth waveform of a desired frequency from a number of sawtooth
waveforms formed by the sawtooth generator 50 in response to the
resolution, and a comparator circuit 54 for outputting the ON signal for
switching the semiconductor laser 34 ON, when the voltage of the sawtooth
waveform output from the waveform selecting circuit 52 is higher than the
voltage of the electric signal output from the D/A converter 48. And with
the above structure, the ON signal having a length in response to the
image density of the original is output.
As shown FIG. 2, the rotary developing device 24 is in a cylindrical form,
and is composed of 4 inverting development-type and two-component
development-type developing sections 56 (56A.about.56D) for Yellow, Cyan,
Magenta, and Black. FIG. 5 shows a schematic diagram of the developing
section 56. The developing section 56 includes a developer housing 57 in a
fan shape in which an opening 57A is formed along the axial direction on
the outer periphery, a magnetic roll 62 comprising a number of fixed
magnets 58 (58A.about.58E) arranged radially and a developing sleeve 60
which rotates in the direction of an arrow B around the fixed magents 58,
a bias power feed 64 for feeding direct-current superposed
alternating-current bias voltage to the developing sleeve 60 to inhibit
the toner from adhering to the white portion, a trimmer bar 66 arranged
upstream of the opening section 57A in the rotating direction for making
the thickness of the magnetic brush comprising the developer constant,
screw augers 68A and 68B arranged downward of the magnetic roll 62 for
stirring the developer, a partition wall 70 arranged between screw augers
68A and 68B and provided with an opening (not shown) at the end portion,
and a toner feed unit (not shown) for feeding the toner to a certain
portion in the screw auger 68B in the development housing 57, in which the
magnetic roll 62, screw augers 68A and 68B, trimmer bar 66, toner feed
unit and partition wall 70 are housed in the development housing 57.
The magnetic roll 62 is so mounted that the axial direction becomes
parallel to the axial direction of the photosensitive material 16, and the
rotary developing device 24 is so arranged that when the opening 57A of
the development housing 57 of each developing sections 56 (A.about.D) is
arranged at a position opposite to the photosensitive material 16, a
predetermined clearance is formed between the magnetic roll 62 included in
the development section 56 and the photosensitive material 16.
Furthermore, a number of fixed magnets 58A.about.E are so arranged that the
polarity of the abutting fixed magnets 58B and 58C arranged downstream of
the opening 57A in the rotation direction becomes the same, and the
polarity of the other abutting fixed magnets 58C and 58D, 58D and 58E, 58E
and 58A, and 58A and 58B becomes different. And the magnetic brush adhered
to the magnetic roll 62 by the pull of the fixed magnets 58C and 58D
arranged on top of the screw auger 68A, is transferred to the opening 57A
of the development housing 57 by the pull of the fixed magnets 58D and 58E
and fixed magnets 58E and 58A and the rotation of the magnetic roll 62, to
rub the photosensitive material 16 (development), as well as the toner
remaining on the magnetic roll 62 is removed from the magnetic roll 62 by
means of the repulsive force of the fixed magnets 58B and 58C, and falls
downward of the development housing 57.
Furthermore, the rotation directions of the screw augers 68A and 68B are
made opposite to each other, and the developer is delivered at the opening
(not shown) formed at the end of the partition wall 70, thereby the fed
toner and the carrier are mixed well to form the developer, which is fed
to the magnetic roll 62.
The rotary developing device 24 having the above-mentioned structure is
connected to a driving device (not shown) connected to the control section
12, and rotated intermittently based on the signal from the control
section 12. Thereby, every time a latent image for each color is formed,
the latent image is developed with a toner of the corresponding color.
As shown in FIG. 2, the transfer device 26 includes a transfer drum 72
rotating in the direction of an arrow C. This transfer drum 72 is so
arranged that the axial direction thereof is parallel to the axial
direction of the photosensitive material 16, and a predetermined clearance
is formed between the photosensitive material 16 and the transfer drum 72.
In addition, in the periphery of the transfer drum 72, there are arranged
a charging device for adsorbing the recording material 78 which is
arranged upstream in the rotation direction of the transfer section 74
where the transfer drum 72 is close to the photosensitive material 16, and
charges the transfer drum 72 in order to adsorb the recording material
transported from the transport passage 76A; a charging device for transfer
80 arranged in the vicinity of the transfer section 74 for transferring
the toner image on the photosensitive material 16 to the recording
material adsorbed on the transfer drum 72; a charging device for peeling
82 arranged downstream of the transfer-charging device 80 in the rotation
direction for charging the transfer drum 72 in order to peel off the
adsorbed recording material; a peel-off nail 84 arranged downstream of the
charging device for peeling 82 in the rotation direction for peeling off
the recording material from the transfer drum 72; and a change eraser 86
arranged downstream of the peel-off nail 84 in the rotation direction for
removing the charge remaining on the transfer drum 72.
The fixing device 32 is arranged on the top of the transport passage 76B
and downstream of the peel-off nail 84 in the direction of transportaion,
and includes a pair of fixing rolls 88A and 88B which put the transport
passage 76B between them. At least one of the pair of fixing rolls 88A and
88B is heated by a heater (not shown), and the recording material
transported from the transfer device 26 is guided to the nipping portion
of the pair of fixing rolls 88A and 88B, and heated by this nipping
portion to fix a multi-color image on the recording material.
A tray 90 is provided downstream of the fixing rolls 88A and 88B in the
direction of transportation, and the recording material on which an image
is fixed is guided to this tray 90 by means of the rotation of the fixing
rolls 88A and 88B.
With the above-mentioned image forming apparatus 10, the original is read
by the original reading section 14, thereby image signals for each color
are formed, and the formed image signals for each color are output
sequentially to the light-beam scanning device 22. On the other hand, the
photosensitive material 16 is charged, latent images for each color are
formed on the photosensitive material 16 by the light-beam scanning device
22, and every time the latent image for each color is formed, the rotary
developing device 24 develops the latent image with a toner of the
corresponding color. The developed toner image for a particular color is
transferred onto the recording material adsorbed by the transfer drum 72.
By repeating the above-mentioned formation of latent images, development,
and transfer for each color onto one recording material, a multi-color
image is formed on the recording material. The recording material on which
a multi-color image is formed is transported to the fixing device 32 for
fixing, and finally transported to the tray 90.
Incidentally, the latent image formed by the image forming apparatus 10 is
put into a binary value, therefore the following description is for the
latent image put into a binary value.
FIG. 6 shows an exposure energy profile on the photosensitive material when
the light-beam spot diameter dB (mm) is made constant, the value of D
expressed by a ratio of a distance dP (mm) between picture elements
abutting in the scanning direction (see FIG. 7) and a beam spot diameter
dB (dB/dP) is designated as 1/1, 1/2 and 1/3, respectively, and a
photosensitive material is exposed at an input image area factor of 10%,
20% and 50% by using a light-beam scanning device. As is seen from FIG. 6,
as the D value grows from 1/3 through to 1/1, the contrast of the exposure
energy profile decreases becoming analogous.
FIG. 8A and 8B show the light potential damping characteristics of the
photosensitive material, and FIG. 9A is a result of the calculation for
determining the surface potential profiles of the photosensitive material
when a photosensitive material having the light potential damping
characteristics shown in FIG. 8A is exposed at an input image area factor
of 50%, and the D value is changed in the exposure energy profile as shown
in FIG. 6. The calculation method is described in, for example,
"Proceedings IS&T's 9th International Congress on Advances in Non-Impact
Printing Technologies, Vol. 9", pp. 97.about.100, published in 1993.
As is seen from FIG. 9A, as the D value becomes larger, the contrast of the
exposure energy profile decreases, with the contrast of the surface
potential profile of the latent image being decreased.
In this specification, the latent image is in a binary state, which means
that the latent image contrast potential .vertline.Va-Vb.vertline. which
is formed by the photosensitive material surface potential Va of the
exposed portion (to be exposed originally) and the photosensitive material
surface potential Vb of the non-exposed portion (not to be exposed
originally), when an exposure at the input image area factor of 50% is
performed, is not less than 90% of the latent image contrast potential
.vertline.Vh-Vl.vertline. which is formed by the charging potential Vh of
the photosensitive material and the average photosensitive material
surface potential Vl of the exposed portion, when an exposure at the input
image area factor of 100% is performed.
Therefore, when a photosensitive material having the light potential
damping characteristics shown in FIG. 8A is used, the latent image can be
put into a binary value by decreasing the D value to less than 1/2.
Furthermore, when a photosensitive material having the light potential
damping characteristics shown in FIG. 8B is used, the latent image can be
put into a binary value by adjusting the exposure energy properly, even if
the D value is 1, as shown in FIG. 9B.
EXAMPLES
The present invention will now be described specifically with reference to
Examples and Comparative Examples. Incidentally, the measurement of the
electric resistance in the Examples and Comparative Examples were all
performed in an environment of a temperature of 22.degree. C. and a
humidity of 55%.
Preparation of the Carrier
Carrier A (to be used in Examples 1, 7 and 13)
______________________________________
Magnetite (MX030A, average particle
100 parts by weight
diameter 50 .mu.m, produced by FDK Corp.)
Toluene 13.5 parts by weight
Styrene-methyl methacrylate copolymer
1.8 parts by weight
(copolymerization ratio
20:80, weight average molecular
weight 50,000)
Potassium titanate coated with
0.9 parts by weight
antimony doped tin oxide
(Dentole BK-100, electric resistance
10.sup.4 .OMEGA. .multidot. cm, fiber length
15 .mu.m, fiber diameter 0.3 .mu.m,
aspect ratio 50, produced by Otsuka
Chemical Co., Ltd.)
______________________________________
The above components except magnetite were dispersed for 1 hour by a sand
mill to prepare a solution for forming a resin coating layer. Then, this
solution for forming a resin coating layer and magnetite were put into a
vacuum degassing type kneader, and stirred for 20 minutes while
decompressing at a temperature of 60.degree. C. to form a resin coating
layer, thus a carrier A was obtained. The thickness of the resin coating
layer was 0.9 .mu.m. Furthermore, the content of the conductive powder was
12% by volume. When this carrier was observed with a scanning electron
microscope, it was confirmed that the carrier had no exposed face and was
uniformly coated.
In addition, the solution for forming the resin coating layer was coated on
an ITO conductive glass substrate so that the thickness thereof became 0.9
.mu.m by using an applicator, to obtain a sample for measuring the
electric resistance of the resin coating film.
The electric resistances of the magnetite and the carrier A in a form of a
magnetic brush were measured, and the electric resistance values when
being extrapolated up to an electric field of 10.sup.4 V/cm were
4.times.10.sup.-5 .OMEGA..cm and 8.times.10.sup.7 .OMEGA..cm,
respectively. In addition, the electric resistance value of the resin
coating film in an electric field of 100 V/cm was 2.times.10.sup.5
.OMEGA..cm.
Carrier B (to be used in Examples 2 and 8)
______________________________________
Ferrite (MF-35, average particle diameter
100 parts by weight
35 .mu.m, produced by Powdertec Co., Ltd.)
Toluene 22 parts by weight
Styrene-methyl methacrylate copolymer
3 parts by weight
(copolymerization ratio 20:80,
weight average molecular weight 50,000)
Titanium oxide coated with antimony
1.3 parts by weight
doped tin oxide (FT-2000, electric resistance
10.sup.1 .OMEGA. .multidot. cm, fiber length 8 .mu.m,
fiber diameter 0.1 .mu.m, aspect ratio 80,
produced by Ishihara Sangyo Kaisha, Ltd.)
______________________________________
The above components except ferrite were dispersed for 1 hour by a sand
mill to prepare a solution for forming a resin coating layer. Then, this
solution for forming a resin coating layer and ferrite were put into a
vacuum degassing type kneader, and stirred for 20 minutes while
decompressing at a temperature of 60.degree. C. to form a resin coating
layer, thus a carrier B was obtained. The thickness of the resin coating
layer was 0.9 .mu.m. Furthermore, the content of the conductive powder in
the resin coating layer was 8% by volume. When this carrier was observed
with a scanning electron microscope, it was confirmed that the carrier had
no exposed face and was uniformly coated.
In addition, the solution for forming the resin coating layer was coated on
an ITO conductive glass substrate so that the thickness thereof became 0.9
.mu.m by using an applicator, to obtain a sample for measuring the
electric resistance of the resin coating film.
The electric resistances of the ferrite and the carrier B in a form of a
magnetic brush were measured, and the electric resistance value when being
extrapolated up to an electric field of 10.sup.4 V/cm were
5.times.10.sup.-2 .OMEGA..cm and 2.times.10.sup.7 .OMEGA..cm,
respectively. In addition, the electric resistance value of the resin
coating film in an electric field of 100 V/cm was 1.times.10.sup.3
.OMEGA..cm.
Carrier C (to be used in Examples 3 and 9)
______________________________________
Ferrite (C28-FB, average particle diameter
100 parts by weight
50 .mu.m, produced by FDK Corp.)
Toluene 14 parts by weight
Styrene-methyl methacrylate copolymer
4 parts by weight
(copolymerization ratio 20:80, weight
average molecular weight 50,000)
Aluminum borate coated with antimony doped
0.8 parts by weight
tin oxide (Pastran 5110S, electric resistance
10.sup.1 .OMEGA. .multidot. cm, fiber length
15 .mu.m, fiber diameter 1 .mu.m, aspect ratio 15,
produced by Mitsui Mining & Smelting Co., Ltd.)
______________________________________
The above components except ferrite were dispersed for 1 hour by a sand
mill to prepare a solution for forming a resin coating layer. Then, this
solution for forming a resin coating layer and ferrite were put into a
vacuum degassing type kneader, and stirred for 20 minutes while
decompressing at a temperature of 60.degree. C. to form a resin coating
layer, thus a carrier C was obtained. The thickness of the resin coating
layer was 1.8 .mu.m. Furthermore, the content of the conductive powder in
the resin coating layer was 5% by volume. When this carrier was observed
with a scanning electron microscope, it was confirmed that the carrier had
no exposed face and was uniformly coated.
In addition, the solution for forming the resin coating layer was coated on
an ITO conductive glass substrate so that the thickness thereof became 1.8
.mu.m by using an applicator, to obtain a sample for measuring the
electric resistance of the resin coating film.
The electric resistances of the ferrite and the carrier C in the form of a
magnetic brush were measured, and the electric resistance values when
being extrapolated up to an electric field of 10.sup.4 V/cm were
1.times.10.sup.-5 .OMEGA..cm and 4.times.10.sup.5 .OMEGA..cm,
respectively. In addition, the electric resistance value of the resin
coating film in an electric field of 100 V/cm was 8.times.10.sup.3 106
.cm.
Carrier D (to be used in Examples 4 and 10)
______________________________________
Iron powder (TSV, average particle diameter
100 parts by weight
60 .mu.m, produced by Powdertec Co., Ltd.)
Toluene 8 parts by weight
Styrene-methyl methacrylate copolymer
1 part by weight
(copolymerization ratio 20:80, weight
average molecular weight 50,000)
Potassium titanate coated with antimony doped
0.3 part by weight
tin oxide (Dentol BK-100, electric resistance
10.sup.4 .OMEGA. .multidot. cm, fiber length 15 .mu.m, fiber
diameter 0.3 .mu.m, aspect ratio 50,
produced by Otsuka Chemical Co., Ltd.)
______________________________________
The above components except iron powder were dispersed for 1 hour by a sand
mill to prepare a solution for forming a resin coating layer. Then, this
solution for forming a resin coating layer and iron powder were put into a
vacuum degassing type kneader, and stirred for 20 minutes while
decompressing at a temperature of 60.degree. C. to form a resin coating
layer, thus a carrier D was obtained. The thickness of the resin coating
layer was 0.9 .mu.m. Furthermore, the content of the conductive powder in
the resin coating layer was 8% by volume. When this carrier was observed
with a scanning electron microscope, it was confirmed that the carrier had
no exposed face and was uniformly coated.
In addition, the solution for forming the resin coating layer was coated on
an ITO conductive glass substrate so that the thickness thereof became 0.9
.mu.m by using an applicator, to obtain a sample for measuring the
electric resistance of the resin coating film.
The electric resistances of the iron powder and the carrier D in the form
of a magnetic brush were measured, and the electric resistance values when
being extrapolated up to an electric field of 10.sup.4 V/cm, were
1.times.10.sup.-14 .OMEGA..cm and 2.times.10.sup.6 .OMEGA..cm,
respectively. In addition, the electric resistance value of the resin
coating film in an electric field of 100 V/cm was 5.times.10.sup.6
.OMEGA..cm.
Carrier E (to be used in Examples 5 and 11)
______________________________________
Magnetite (MX030A, average particle
100 parts by weight
diameter 50 .mu.m, produced by FDK Corp.)
Toluene 13.5 parts by weight
Styrene-methyl methacrylate copolymer
1.6 parts by weight
(copolymerization ratio 20:80,
weight average molecular weight 50,000)
Potassium titanate coated with antimony doped
1.6 parts by weight
tin oxide (Dentol BK-100, electric resistance
10.sup.4 .OMEGA. .multidot. cm, fiber length 15 .mu.m, fiber
diameter 0.3 .mu.m, aspect ratio 50, produced
by Otsuka Chemical Co., Ltd.)
______________________________________
The above components except magnetite were dispersed for 1 hour by a sand
mill to prepare a solution for forming a resin coating layer. Then, this
solution for forming a resin coating layer and magnetite were put into a
vacuum degassing type kneader, and stirred for 20 minutes while
decompressing at a temperature of 60.degree. C. to form a resin coating
layer, thus a carrier E was obtained. The thickness of the resin coating
layer was 0.9 .mu.m. Furthermore, the content of the conductive powder in
the resin coating layer was 22% by volume. When this carrier was observed
with a scanning electron microscope, it was confirmed that the carrier had
no exposed face and was uniformly coated.
In addition, the solution for forming the resin coating layer was coated on
an ITO conductive glass substrate so that the thickness thereof became 0.9
.mu.m by using an applicator, to obtain a sample for measuring the
electric resistance of the resin coating film.
The electric resistance of the carrier E in the form of a magnetic brush
was measured, and the electric resistance value when being extrapolated up
to an electric field of 10.sup.4 V/cm was 1.times.10.sup.5 .OMEGA..cm. In
addition, the electric resistance value of the resin coating film in an
electric field of 100 V/cm was 2.times.10.sup.3 .OMEGA..cm.
Carrier F (to be used in Comparative Examples 1, 4 and 6)
______________________________________
Ferrite (C28-FB, average particle diameter
100 parts by weight
50 .mu.m, produced by FDK Corp.)
Toluene 14.5 parts by weight
Styrene-methyl methacrylate copolymer
2 parts by weight
(copolymerization ratio 20:80,
weight average molecular weight 50,000)
______________________________________
A solution for forming a resin coating layer obtained by dissolving a resin
in toluene, and ferrite were put into a vacuum degassing type kneader, and
stirred for 20 minutes while decompressing at a temperature of 60.degree.
C. to form a resin coating layer, thus a carrier F was obtained. The
thickness of the resin coating layer was 0.9 .mu.m. When this carrier was
observed with a scanning electron microscope, it was confirmed that the
carrier had no exposed face and was uniformly coated.
In addition, the solution for forming the resin coating layer was coated on
an ITO conductive glass substrate so that the thickness thereof became 0.9
.mu.m by using an applicator, to obtain a sample for measuring the
electric resistance of the resin coating film.
The electric resistance of the carrier F in the form of a magnetic brush
was measured, and the value in an electric field of 10.sup.4 V/cm was
6.3.times.10.sup.10 .OMEGA..cm. Furthermore, the value in an electric
field of 400 V/cm was 1.0.times.10.sup.11 .OMEGA..cm, and the value in an
electric field of 4000 V/cm was 9.8.times.10.sup.10 .OMEGA..cm. In
addition, the electric resistance value of the resin coating film in an
electric field of 100 V/cm, was 1.times.10.sup.13 .OMEGA..cm. As is seen
from this Comparative Example, when a resin having high resistance was
uniformly coated on a core having low resistance, abrupt change in the
electric resistance due to the electric field was not seen.
Carrier G (to be used in Comparative Example 2)
______________________________________
Ferrite (F-300, average particle diameter
100 parts by weight
50 .mu.m, produced by Powdertec Co., Ltd.)
Toluene 12.3 parts by weight
Styrene-methyl methacrylate copolymer
1.6 parts by weight
(copolymerization ratio 20:80,
weight average molecular weight 50,000)
Titanium oxide coated with antimony doped
1.9 parts by weight
tin oxide (FT-2000, electric resistance
10.sup.1 .OMEGA. .multidot. cm, fiber length 8 .mu.m,
fiber diameter 0.1 .mu.m, aspect ratio 80,
produced by Ishihara Sangyo Kaisha Ltd.)
______________________________________
The above components except ferrite were dispersed for 1 hour by a sand
mill to prepare a solution for forming a resin coating layer. Then, this
solution for forming a resin coating layer and ferrite were put into a
vacuum degassing type kneader, and stirred for 20 minutes while
decompressing at a temperature of 60.degree. C. to form a resin coating
layer, thus a carrier G was obtained. The thickness of the resin coating
layer was 0.9 .mu.m. Furthermore, the content of the conductive powder in
the resin coating layer was 20% by volume. When this carrier was observed
with a scanning electron microscope, it was confirmed that the carrier had
no exposed face and was uniformly coated.
In addition, the solution for forming the resin coating layer was coated on
an ITO conductive glass substrate so that the thickness thereof became 0.9
.mu.m by using an applicator, to obtain a sample for measuring the
electric resistance of the resin coating film.
The electric resistance of ferrite and the carrier G in the form of a
magnetic brush were measured, and the values in an electric field of
10.sup.4 V/cm were 9.1.times.10.sup.7 .OMEGA..cm (measured value) and
2.times.10.sup.6 .OMEGA..cm (extrapolated value), respectively. In
addition, the electric resistance value of the resin coating film in an
electric field of 100 V/cm was 5 .OMEGA..cm.
Carrier H (to be used in Comparative Examples 3 and 5)
______________________________________
Ferrite (C28-FB, average particle diameter
100 parts by weight
50 .mu.m, produced by FDK Corp.)
Toluene 12.6 parts by weight
Styrene-methyl methacrylate copolymer
1.3 parts by weight
(copolymerization ratio 20:80,
weight average molecular weight 50,000)
Barium sulfate coated with oxygen deficit
3.6 parts by weight
type tin oxide (Pastran 4320, electric resistance
10.sup.2 .OMEGA. .multidot. cm, spherical shape,
particle diameter 0.1 .mu.m, produced by
Mitsui Mining & Smelting Co., Ltd.)
______________________________________
The above components except ferrite were dispersed for 1 hour by a sand
mill to prepare a solution for forming a resin coating layer. Then, this
solution for forming a resin coating layer and ferrite were put into a
vacuum degassing type kneader, and stirred for 20 minutes while
decompressing at a temperature of 60.degree. C. to form a resin coating
layer, thus a carrier H was obtained. The thickness of the resin coating
layer was 0.9 .mu.m. Furthermore, the content of the conductive powder in
the resin coating layer was 35% by volume. When this carrier was observed
with a scanning electron microscope, it was confirmed that the carrier had
no exposed face and was uniformly coated.
In addition, the solution for forming the resin coating layer was coated on
an ITO conductive glass substrate so that the thickness thereof became 0.9
.mu.m by using an applicator, to obtain a sample for measuring the
electric resistance of the resin coating film.
The electric resistance of the carrier H in the form of a magnetic brush
was measured, and the electric resistance value when being extrapolated up
to an electric field of 10.sup.4 V/cm was 2.times.10.sup.5 .OMEGA..cm. In
addition, the electric resistance value of the resin coating film in an
electric field of 100 V/cm was 1.times.10.sup.4 .OMEGA..cm.
Carrier I (to be used in Comparative Examples 10 and 16)
______________________________________
Ferrite (MF-35, average particle diameter
100 parts by weight
35 .mu.m, produced by Powdertec Co., Ltd.)
Toluene 22 parts by weight
Styrene-methyl methacrylate copolymer
3 parts by weight
(copolymerization ratio 20:80,
weight average molecular weight 50,000)
Titanium oxide coated with oxygen deficit
1.3 parts by weight
type tin oxide (custom made product,
electric resistance 10.sup.1 .OMEGA. .multidot. cm,
fiber length 8 .mu.m, fiber diameter 0.1 .mu.m,
aspect ratio 80, producedby Mitsui Mining
& Smelting Co., Ltd.)
______________________________________
The above components except ferrite were dispersed for 1 hour by a sand
mill to prepare a solution for forming a resin coating layer. Then, this
solution for forming a resin coating layer and ferrite were put into a
vacuum degassing type kneader, and stirred for 20 minutes while
decompressing at a temperature of 60.degree. C. to form a resin coating
layer, thus a carrier I was obtained. The thickness of the resin coating
layer was 0.9 .mu.m. Furthermore, the content of the conductive powder in
the resin coating layer was 8% by volume. When this carrier was observed
with a scanning electron microscope, it was confirmed that the carrier had
no exposed face and was uniformly coated.
In addition, the solution for forming the resin coating layer was coated on
an ITO conductive glass substrate so that the thickness thereof became 0.9
.mu.m by using an applicator, to obtain a sample for measuring the
electric resistance of the resin coating film.
The electric resistances of the ferrite and the carrier I in the form of a
magnetic brush were measured, and the electric resistance values when
being extrapolated up to an electric field of 10.sup.4 V/cm were
5.times.10.sup.-2 .OMEGA..cm, and 6.times.10.sup.7 .OMEGA..cm,
respectively. In addition, the electric resistance value of the resin
coating film in an electric field of 100 V/cm was 3.times.10.sup.3
.OMEGA..cm.
Preparation of a Toner Used in Examples 1 to 6 and Comparative Examples 1
to 3
______________________________________
Linear polyester resin 100 parts by weight
(linear polyester obtained from
terephthalic acid/bisphenol
A ethylene oxide adduct/
cyclohexanedimethanol;
Tg = 62.degree. C.,
Mn = 4,000,
Mw = 12,000,
acid value = 12,
hydroxyl value = 25)
Magenta pigment (C. I. Pigment, Red 57)
3 parts by weight
______________________________________
The above mixture was kneaded in an extruder, and pulverized with a jet
mill. Thereafter, the mixture was dispersed with a wind energy classifier
to obtain a Magenta toner of d50=7 .mu.m.
Preparation of a Developer
100 parts by weight of the afore-mentioned carrier A.about.I were mixed
with 8 parts by weight of the above Magenta toner, to prepare developers
for Examples 1 to 10 and Comparative Examples 1 to 3.
Evaluation Test
The above developers were subjected to a reproduction test by using an
electrophotographic copying machine (produced by Fuji Xerox Co., A-Color
630) and adjusting the evaluation environment to a low temperature and low
humidity (10.degree. C., 15%), a normal temperature and normal humidity
(22.degree. C., 55%) and a high temperature and high humidity (28.degree.
C., 85%), respectively.
Image Density
A solid image (20 mm.times.20 mm) having an original density of 0.50 was
reproduced, and the relative reflection density of the output image with
respect to the white paper was measured by a Macbeth densitometer. It was
judged that the closer the image density was to 0.50, the better it was.
The first copy and the 50,000th copy were evaluated at normal temperature
and normal humidity, and the first copy was evaluated under low
temperature and low humidity, and high temperature and high humidity.
Density Unevenness
The output image was evaluated by visual inspection, by providing a
criteria. .largecircle. indicates that the density is uniform, and X
indicates that the density is not uniform. The evaluation was performed
for the first copy.
Brush Mark
The number of white marks occurring in the output image was evaluated by a
microscope with respect to unit lengths (5 mm) in the brush direction and
in the right-angle direction. The first copy was subjected to the
evaluation.
Carrier-Over
The output image was evaluated by visual inspection. .largecircle.
indicates that no carrier-over is seen, and X indicates that carrier-over
is recognized. The first copy was subjected to the evaluation.
The above results are shown in Table 1.
TABLE 1
__________________________________________________________________________
Brush Mark
Image Density
Density Unevenness
(number/5 mm)
Carrier-over
Low High Low High Low High Low High
temperature/
temperature/
temperature/
temperature/
temperature/
temperature/
temperature/
temperature/
low high low high low high low high
Carrier humidity
humidity
humidity
humidity
humidity
humidity
humidity
humidity
__________________________________________________________________________
Example 1
A 0.50 0.53 .largecircle.
.largecircle.
0 0 .largecircle.
.largecircle.
Example 2
B 0.51 0.53 .largecircle.
.largecircle.
0 0 .largecircle.
.largecircle.
Example 3
C 0.52 0.53 .largecircle.
.largecircle.
0 0 .largecircle.
.largecircle.
Example 4
D 0.51 0.53 .largecircle.
.largecircle.
0 0 .largecircle.
.largecircle.
Example 5
E 0.52 0.60 .largecircle.
.largecircle.
0 0 .largecircle.
.largecircle.
Comparative
F 0.42 0.45 X X 0 0 .largecircle.
.largecircle.
Example 1
Comparative
G 0.43 0.46 X X 8 12 X X
Example 2
Comparative
H 0.52 0.82 .largecircle.
.largecircle.
0 0 .largecircle.
.largecircle.
Example 3
Example 8
I 0.51 0.58 .largecircle.
.largecircle.
0 0 .largecircle.
.largecircle.
__________________________________________________________________________
As can be seen from this Table, when the carriers of the present invention,
particularly the carriers A, B, C and D were used, excellent image quality
could be obtained, and they were stable against the environmental changes.
With respect to carrier E, since the added amount of the conductive powder
was relatively large, the charging quantity decreased at high temperature
and high humidity, and the image density was somewhat high. Furthermore,
in carrier I, since the core material surface of the conductive powder was
coated with oxygen deficit type tin oxide, the charging quantity decreased
at high temperature and high humidity, and the image density was somewhat
high.
On the other hand, when a core having low resistance was uniformly coated
with a resin having high resistance, as in carrier F of the Comparative
Example, brush marks and carrier-over were not seen, but density
unevenness was seen in the central portion and peripheral portion of the
solid image, and the image density was low. It is considered that this is
because the electric resistance of the resin coating layer is too high to
keep the electric resistance of the carrier higher within a predetermined
value, hence IMB type characteristics appear. When a resin coating layer
having low resistance was formed on the core having high resistance, as in
carrier G of the Comparative Example, brush marks and carrier-over
occurred, the image density was low and density unevenness was seen. When
a resin coating layer having medium resistance was formed on the core
having low resistance, as in carrier H of the Comparative Example, since
the spherical conductive powder was used in a large quantity, brush marks,
carrier-over and density unevenness were not seen, but the image density
became high at high temperature and high humidity.
As can be understood from the above results, color images having high
quality can be stably obtained without image defects, by uniformly forming
a resin coating layer with medium resistance containing acicular or
fibrous conductive powder on a core having low resistance and controlling
the electric resistance of the carrier within a desired range.
Preparation of the Developer Used in Examples 7 to 12 and Comparative
Examples 4 to 5
100 parts by weight of the above-mentioned carrier A.about.I (except
carrier G) were mixed with 5 parts by weight of Magenta toner for A-Color
635 produced by Fuji Xerox Co. to prepare developers for Examples 7 to 12
and Comparative Examples 4 to 5.
Developing Device and Evaluation Test
The above developers were put in the image forming apparatus 10 shown in
FIG. 2, and the evaluation environment was adjusted to low temperature and
low humidity (10.degree. C., 15%) and high temperature and high humidity
(28.degree. C., 85%), respectively, to evaluate the saturation area, image
density, brush marks and carrier-over.
Incidentally, the actual developing condition and the evaluation method are
as follows.
Developing Conditions
Type of photosensitive material 16: OPC (84 mm.O slashed.)
Process speed: 160 mm/s
Initial charging potential: -650 V
Exposed portion Potential: -200 V
ROS: LED (400 dpi)
Outer diameter of magnetic roll 62: 30 mm.O slashed.
Peak value of the magnetic flux density in the radial direction of magnetic
roll 62: 100 mT
Rotation speed of magnetic roll 62: 336 mm/s
Interval (DRS) between photosensitive material 16 and developer carrier 60
when developing section 56 faces photosensitive material 16: 0.5 mm
Saturation Area
The developing bias potential was sequentially changed, and .largecircle.
indicates that a saturation area was seen in the development curve
expressed by the contrast potential and the developing toner quantity, and
X indicates that a saturation area was not seen therein.
Image Density
A solid image (20 mm.times.20 mm) having the original density of 1.80 was
reproduced, the relative reflection density of the output image with
respect to the white paper was measured by a Macbeth densitometer. It was
judged that the closer the image density was to 1.80, the better it was.
The first copy was subjected to the evaluation.
Brush Marks and Carrier-Over
The evaluation was done in the same manner as in Examples 1 to 6, and
Comparative Examples 1 to 3.
Incidentally, the developing bias potential in the evaluation of the image
density, brush marks and carrier-over was applied to the magnetic roll 62
so that the developing toner quantity showed the saturation characteristic
with respect to the developer having a saturation area. Specifically, the
direct-current superposed alternating-current bias potential having the DC
component of -500 V and the AC component (voltage between peaks) of 100 V
(6 kHz) was used.
The above results are shown in Table 2.
TABLE 2
__________________________________________________________________________
Brush Marks
Image Density
Saturation Area
(number/5 mm)
Carrier-over
Low High Low High Low High Low High
temperature/
temperature/
temperature/
temperature/
temperature/
temperature/
temperature/
temperature/
low high low high low high low high
Carrier humidity
humidity
humidity
humidity
humidity
humidity
humidity
humidity
__________________________________________________________________________
Example 7
A 1.80 1.83 .largecircle.
.largecircle.
0 0 .largecircle.
.largecircle.
Example 8
B 1.81 1.83 .largecircle.
.largecircle.
0 0 .largecircle.
.largecircle.
Example 9
C 1.82 1.83 .largecircle.
.largecircle.
0 0 .largecircle.
.largecircle.
Example 10
D 1.81 1.83 .largecircle.
.largecircle.
0 0 .largecircle.
.largecircle.
Example 11
E 1.78 1.90 .largecircle.
.largecircle.
0 0 .largecircle.
.largecircle.
Comparative
F 1.62 1.65 X X 0 0 .largecircle.
.largecircle.
Example 4
Comparative
H 1.62 2.01 .largecircle.
.largecircle.
0 0 .largecircle.
.largecircle.
Example 5
Example 12
I 1.79 1.88 .largecircle.
.largecircle.
0 0 .largecircle.
.largecircle.
__________________________________________________________________________
As can be understood from this Table, when the carriers of the present
invention, particularly the carriers A, B, C and H were used, the
saturation area could be seen, brush marks and carrier-over were not seen
and the image density was stable against the environmental changes. With
respect to carrier E, since the added amount of the conductive powder was
relatively large, the charging quantity decreased at high temperature and
high humidity, and the image density was somewhat high. Furthermore, in
carrier I, since the core material surface of the conductive powder was
coated with oxygen deficit type tin oxide, the charging quantity decreased
at high temperature and high humidity, and the image density was somewhat
high. On the other hand, when the core having low resistance was uniformly
coated with a resin having high resistance, as in carrier F of the
Comparative Example, brush marks and carrier-over were not seen, but the
saturation area could not be seen, the density unevenness was seen in the
central portion and peripheral portion of the solid image, and the image
density was low. It is considered that this is because the electric
resistance of the resin coating layer is too high, keeping the electric
resistance of the carrier higher than the predetermined value, hence IBM
type characteristics appear. When a resin coating layer having medium
resistance was formed on a core having low resistance, as in carrier H of
the Comparative Example, the saturation area could be obtained, and brush
marks and carrier-over were not seen, but since the spherical conductive
powder was used in a large quantity, the image density became high at high
temperature and high humidity.
Example 14
100 parts by weight of the above-mentioned carrier A was mixed with 5 parts
by weight of Yellow toner, Magenta toner and Cyan toner for A-Color 635
produced by Fuji Xerox Co., respectively, to prepare developers for the
respective colors, and they were put into each developing section 56 of
the image forming apparatus 10 shown in FIG. 2. In addition, as the
photosensitive material 16, a photosensitive material having a light
damping curve as shown in FIG. 8B and having a sensitivity unevenness in
the peripheral direction (photosensitive material A) was used, and an
image corresponding to a substantially flesh tint (the input image area:
50%) was output. In addition, as the photosensitive material 16, a
photosensitive material having a light damping curve as shown in FIG. 8A
and having a sensitivity unevenness in the axial direction of the rotation
(photosensitive material B) was used, and an image corresponding to a
substantially flesh tint (the input image area: 50%) was output. At this
time, the direct-current superposed alternating-current bias potential
having the DC component of -500 V and the AC component (voltage between
peaks) of 100 V (6 kHz) was applied to the magnetic roll 62, so that the
developing toner quantity developed on the latent image showed the
saturation characteristic.
Comparative Example 6
100 parts by weight of the above-mentioned carrier F was mixed with 5 parts
by weight of Yellow toner, Magenta toner and Cyan toner for A-Color 635
produced by Fuji Xerox Co., respectively, to prepare developers for the
respective colors, and the test was conducted as in the above Example.
Incidentally, in Example 13 and Comparative Example 6, the latent image
contrast potential at the time of exposure with the input image area
factor of 50%, was 90% or higher of the latent image contrast potential
formed by the charging potential of the photosensitive material and the
surface potential at the time of exposure with the input image area factor
of 100%.
The difference in color in the image surface on the recording material
corresponding to the whole surface of the image forming area of the
photosensitive material was determined by visual inspection. The
evaluation result is shown by .largecircle. when there is no difference in
color in the same surface, and X when there is difference in color in the
same surface. The result is shown in Table 3.
TABLE 3
______________________________________
Photosensitive
Photosensitive
Carrier material A material B Notes
______________________________________
Example 13
A .smallcircle.
.smallcircle.
Saturation
phenomenon
Comparative
F .smallcircle.
X Non-saturation
Example 6 phenomenon
______________________________________
In the case of a developer containing the carrier F of the Comparative
Example, that is, a developer which could not obtain the saturation area,
the difference in color in the image surface due to the sensitivity
unevenness of the photosensitive material could be seen clearly. But in
the case of a developer containing the carrier A of the Example, that is,
a developer which could obtain the saturation area, it was found that the
difference in color in the image surface due to the sensitivity unevenness
of the photosensitive material was not seen, and the developer showed
stable gradation against the change of the potential of the latent image.
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